a few tweaks to SetEnvIfExpr doc

From a question on IRC, show a backreference example.
Fix a bad copy/paste where a link to <if> was present.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1909833 13f79535-47bb-0310-9956-ffa450edef68
1 file changed